Hey everyone,
I just asked a professor this same question and I found that yes it's possible. Even when you sleep your neurons are still active! And since the mind can't tell the difference between physical reality and mental reality, your mental reality does affect your neurons just like how your physical reality would. From this you can build long term relationships within the brain even while you sleep through Law of Reinforcement. So I suppose if you can control your dream enough and play violin for four hours within a dream, your neurons will continue to have a longer and stronger relationship. So four hours of practice within a dream would be the same as four hours of practice in the physical reality!!!!
